1. Field of the Invention
This invention relates to an electronic musical instrument, and more particularly to an electronic musical instrument of the system in which frequencies related to those of sounds produced are provided by a time sharing control in channels provided corresponding to a limited number of sounds to be produced simultaneously.
2. Description of the Prior Art
Heretofore, there have been proposed electronic musical instruments of the digital system in U.S. Pat. No. 3,515,792 entitled "Digital Organ" and in U.S. Pat. No. 3,809,786 entitled "Computor Organ". In the former, one cycle of a required musical waveform is quantized by sampling and stored in a read-only memory and the content of the read-only memory is read out repetitiously by one or more clocks corresponding to a keyboard or keyboards and multiplied or divided by an envelope waveshape stored in the read-only memory. In the latter, a discrete Fourier algorithm is implemented to compute each amplitude from a stored set of harmonic coefficients C.sub.n and a selected frequency member R. In more detail, the computations occur at regular time intervals independent of the waveshape period and the waveshape sample point qR (q=1, 2, 3, . . . ) is computed by a note interval adder from the frequency number R corresponding to the key depressed. Further, W harmonics are read out by a harmonic interval adder from the note interval adder and is multiplied by the stored harmonic coefficients C.sub.n representing features of the musical waveshape to calculate C.sub.n sin(.pi.nqR/W) (n=1, 2, 3, . . . , W). These calculations are carried out in real time, so that the musical waveshape is obtained in real time.
These two methods have the following defects. With the former method, since the musical waveshape is stored in a read-only memory, its stored content is not easy to change and, for obtaining many musical waveshapes, it is necessary to provide many memories corresponding to desired musical waveshapes, respectively. As compared with the above method, the latter method has the advantage that a desired musical waveshape can be synthesized, but since the calculations are achieved in real time, the computor organ of this method requires a very high clock frequency.
For example, for generation of harmonics up to 32nd one with respect to a note having a note frequency of 2.093 KHz (C.sub.7) at the highest, used in the computor organ, the clock frequency required is 4.29 MHz in a single channel. In a polyphonic tone synthesizing system in which note data are time shared by using a single computation channel corresponding to twelve notes, the clock frequency becomes as high as 51.43 MHz. Therefore, integration of this system is difficult and is not advisable from the economical point of view.
In connection with the latter system, there have been proposed various musical waveshape synthesizing methods. But many of these methods usually employ analog processing in combination with digital processing and an error occurs in relation to frequency and the use of a D-C converter and so on leads to an increase in the manufacturing cost of electronic musical instruments.